Metallic nanoparticles have unique properties which differ from those of bulk and atomic species. These unique properties elicit scientific and practical interest.
The difference between metallic nanoparticles and bulk and atomic species is related to differences in electronic structure. The nanoparticles have relatively large surface area with a high percentage of surface atoms. Metal nanoparticles exhibit a drastic decrease in melting point compared to that of the bulk material and/or are characterized by enhanced reactivity of the surface atoms and/or unique optical properties.
Nano-sized materials are acknowledged to have novel properties and applications in electrochemistry, microelectronics, optical, electronic and magnetic devices and sensors, new types of active and/or selective catalysts and biosensors have been previously proposed.
Availability of stable concentrated nano-colloids of metals with low resistivity would create new possibilities in computer-defined direct-write noncontact technologies, such as ink-jet printing, for deposition of metallic structures on various substrates.
Ink-jet is a non-impact dot-matrix printing technology, in which droplets of ink are jetted from a small orifice directly onto a specified position on a substrate to create an image. Today the majority of activities in ink-jet printing are in the drop-on-demand methods. Depending on the mechanism used in the drop formation process, the technology can be categorized into four methods: thermal, piezoelectric, electrostatic, and acoustic ink-jet. Most commercially available drop-on-demand ink-jet printers currently use either thermal or piezoelectric principles
Ink chemistry and formulation represents a major challenge in ink-jet printing. Ink-jet inks must meet strict physico-chemical standards in order to achieve desired performance and reliability of the printing apparatus and/or to achieve acceptable printed patterns. The majority of inks in office and home ink-jet printers are water-based inks. Typical water-based ink-jet inks are composed of a large number of components, aimed at meeting the complex requirements for good printing performance. In particular, in the case of pigment-based inks, the pigment should be in the nanometer size range to prevent sedimentation and orifice clogging.
In addition to conventional graphic applications, ink-jet printing has been applied to various other applications, including fabrication of transistors and organic light emitting diodes, polymer films, structural ceramics and biotechnology. Currently, these applications have not been developed commercially.
A well-known approach to overcome the oxidation problem is based on the protection of nanoparticles with a protecting shell formed by ligands [P. Kanninen, C. Johans, J. Merta, K. Kontturi, J. Coll. Inter. Scl 2008, 318, 88; KR Patent 20060085704 to Choi Soon Lim et al. and Han et al. 2001 Mol. Cryst And Liq. Cryst 371:127-130], polymers [N. Shpaisman, S. Margel, Chem. Mater. 2006, 18, 396.] or silica [W. Fu, H. Yang, L. Chang, M. Li, H. Bala, Q. Yu, G. Zou, Colloids and Surfaces A: Physicochem. Eng. Aspects 2005, 262, 71 and M. Aslam, S. Li, V. P. Dravid, J. Am. Ceram. Soc. 2007, 90, 950.]. The main disadvantage of this approach is the formation of a nonmetallic non-conductive coating. The other approach may be deposition of a thin layer of a protecting noble metal shell, for example Ag, Au or Pt, on the preformed nanoparticles (core nanoparticle), which protect the latter from oxidation, while leaving most of the core properties unchanged. However, it has been reported that such a process may lead to undesirable formation of individual particles of the second metal, in addition or instead of formation of a shell around the preformed core nanoparticles [S. Mandal, K. M. Krishman, J. Mater. Chem. 2007, 17, 312.]. To overcome this drawback, a selective reduction method, which should take place only at the surface of the core particle, is required. For example, copper-silver core shell nanoparticles were formed at 250 C in a hydrocarbon based solvent to form conductive ink [U.S. Patent application publication 2007/0212562]. It is clear that using a hydrocarbon medium for reaction is not favorable due to cost, environmental and health considerations, and performing the reaction at such high temperatures is problematic due to high energy and equipment costs.
WO 2007/140479 describes formation of nanoparticles comprising a core having a largest dimension less than about 10 nm; and a metal layer substantially surrounding the core and having a largest dimension less than about 200 nm using solvents such as polyol, ethylene glycol, propylene glycol, 1,3-propanediol, and combinations thereof. According to this reference, the layer surrounding the core can comprise silver, gold palladium or platinum.